The following description is provided to assist the understanding of the reader. None of the information provided or references cited is admitted to be prior art.
In traditional textile dyeing processes, large amounts of water are used as a dye solvent, resulting in one of the largest sources of water pollution for industrialized countries. An alternative dye solvent to water is super critical carbon dioxide (sc-CO2). In addition to reducing water pollution, sc-CO2 includes several advantages over water. The lower viscosity of sc-CO2 allows for higher diffusion rates and lower mass transfer resistances than water, resulting in increased penetration of the dye into the fiber and decreased dyeing times. Thus, less energy is required for the dyeing process.
In addition, dye and sc-CO2 can be easily separated and reused. In contrast, in the aqueous system, leftover water and dye cannot be easily separated and reused, resulting in large amounts of unusable, polluted water. The use of sc-CO2 results in a dry dyed fabric, and thus does not require extensive drying time as does the aqueous system. Furthermore, the dyeing process using sc-CO2 does not generate carbon dioxide and thus is not a direct contributor to green house gas emissions. In addition, sc-CO2 can be recycled.
Synthetic fibers, such as polyester, can be easily dyed using a sc-CO2 dyeing process. However, the application of the sc-CO2 dyeing process to natural fibers, such as cotton and wool, has been limited thus far. Sc-CO2 is a hydrophobic solvent, which does not swell hydrophilic cotton or wool fibers. Sc-CO2 is unable to break the hydrogen bonds between adjacent molecular chains to disrupt the structure of the natural fibers in order to facilitate the diffusion of the dye into the fibers. In addition, most commercial cotton dyes are salts, which are insoluble in sc-CO2 without the use of co-solvents. However, the use of co-solvents requires that the fabrics become saturated, thus requiring drying of the fabrics and more complex cleaning of the process chamber and dyeing components.